Low-energy Plasma-enhanced Chemical Vapor Research Articles

Germanium: Epitaxy and its applications

This paper reviews the most important properties of germanium, gives an insight into the newer techniques and technology for the growth of epitaxial Ge thin layers and focuses on some applications of this material, with a special emphasis on recent achievements in electronics and photovoltaics. We will highlight the recent development of Ge research and will give an account of the most important Ge applications that emerged in the last two decades. Germanium is a key material in modern material science and society: it is used as a dopant in fiber optic glasses and in semiconductor devices, both in activating conduction in layers and also as a substrate for III–V epitaxy. Ge is also widely used in infrared (IR) detection and imaging and as a polymerization catalyst for polyethylene terephthalate (PET). Moreover, high-speed electronics for cell phone communications relies heavily on SiGe alloys. Ge electronics is nowadays gaining new interest because of the enhanced electronic properties of this material compared to standard silicon devices, but the lack of a suitable gate oxide still limits its development. High efficiency solar cells, mainly for space use but also for terrestrial solar concentration have surpassed 40% efficiency and Ge has a lead role in achieving this goal. The main focus of the paper is on Ge epitaxy. Since epitaxy starts from the surface of the substrate, different studies on substrate pre-epitaxy, surface analysis and preparations are reviewed, covering the most common substrates for Ge deposition such as Ge, Si and GaAs. The most used Ge precursors such as GeH 4 and GeCl 4 are described, but several novel precursors, mostly metal-organic, have recently been developed and are becoming more common in epitaxial Ge deposition. Epitaxial growth of Ge by means of the most common methods, including Chemical Vapour Deposition and Molecular Beam Epitaxy is discussed, along with some recent advances in Ge deposition, such as Atomic Layer Deposition and Low Energy Plasma-Enhanced Chemical Vapour Deposition. Several Ge applications are finally discussed, with the aim of providing insights into the potential of this material for the development of novel devices that are able to surpass the current limits of standard device design. Ge in microelectronics is becoming more and more important, thanks to the possibilities offered by bandgap engineering of strained SiGe/Si. However, lack of a good Ge oxide is posing several problems in device improvement. In the field of photovoltaics Ge is mainly used as a substrate for high efficiency III–V solar cells and for the development of thermophotovoltaic devices instead of the most expensive and scarcer GaSb. In this field, Ge epitaxy is very rare but the development of an epitaxial Ge process may help in developing new solar cells concepts and to improve the efficiency of thermophotovoltaic converters. Ge may play a role even in new spintronics devices, since a GeMn alloy was found to have a higher Curie temperature than GaAsMn.

Plasma Composition and Kinetic Reaction Rates in a LEPECVD Ar-SiH4-H2 Plasma During nc-Si Films Deposition for Photovoltaic Applicationss

The optimal plasma composition for the deposition of nano-crystalline silicon (nc-Si) films is investigated in connection to electron-impact and gas-phase reaction rates for the generation of useful radicals in a low energy plasma-enhanced chemical vapor deposition (LEPECVD) process. Mass spectrometry for the analysis of plasma composition and Langmuir probe space-resolved plasma parameters and electron energy distribution functions (eedf) are evaluated and discussed for Ar-H2 and Ar-SiH4-H2 plasma in the LEPECVD reactor. The observed ion and radical trends during nc-Si deposition can be explained by the dominance of gas-phase rates over electron-impact rates for silane reactions, leading to depletion of SiH3 and SiH2 towards SiH and Si that become precursors of the nc-Si layers.

Defect studies on silicon and silicon–germanium for PV and optoelectronic applications

Defect monitoring and engineering techniques were applied to multicrystalline silicon grown from electronic grade silicon scraps and strained silicon–germanium alloys to test their suitability for low-cost photovoltaics and high-speed electronics, respectively. The combination of lifetime, photoluminescence and electron beam induced current (EBIC) measurements was found a very effective practice for the analysis of the effect of impurities on the electrical activity of grain boundaries and dislocations in multicrystalline silicon. Room temperature photoluminescence has been also used to monitor the effect of various cell fabrication process steps on the local recombination activity of defects. The application of the EBIC technique was found as well a good tool for a quantitative determination of threading dislocation density in Si–Ge alloys, in view of the optimisation of the low energy plasma-enhanced chemical vapour deposition (LEPECVD) used for their growth.